A mixer is an important block of radio frequency (RF) frontend circuits, and has been widely used in transceiver systems. The mixer is used to perform frequency shifting, i.e., to shift the carrier frequency of a desired signal from one frequency to another. For a transmitter, it shifts the desired signal from a low frequency band to a radio frequency band. This process is called up-mixing or up-conversion. For a receiver, it shifts a desired signal from a radio frequency band to a low frequency band. This process is called down-mixing or down-conversion.
The mixer generally performs signal mixing operations based on a local oscillating (LO) signal. In a direct conversion (DC) transceiver (TRX), for the down-conversion operation, the frequency of a received signal is the same as that of the LO signal, while in the up-conversion operation, the frequency of an output signal is the same as that of the LO signal. The LO signal is usually generated by dividing or multiplying the frequency of a voltage control oscillating (VCO) signal.
A tuner is generally used in electronic devices such as radios, TVs, etc., to select a desired channel from channels with different center frequencies. Thus, the tuner is equivalent to a down-mixer with an adjustable mixing frequency. In order to tune for channels of various frequencies, the frequency of a local clock is desired to be adjusted. It is generally desired to design different frequency dividers or frequency multipliers with relatively complicated circuit structures. In addition, when there is a frequency variation for the local clock, it may take a long time to stabilize the system, which is adverse to fast switching of channels.
Thus, there is a need to provide a signal tuner to resolve at least one of the aforesaid problems.